According to criminal history female serial killer Jeanne Weber manually strangled herself. How the heck was it possible?

You can apply a ligature eg a stocking or strip of fabric and pull it by hand - if the knot holds you will throttle yourself. She strangled her victims so it was clearly something she knew how to do. However I think this is not the case here.

Some histories claim she hanged herself, and given that some accounts were written in French this may be a problem in translation. See this from 'Crimefeed' (On July 5, 1910, French serial killer Jeanne Weber, who strangled at least 10 children, including her own, was found hanging from a rope in her cell.)

Many people have committed suicide by strangling themselves. They tie something strong around their neck and leap off a chair or table and strangle themselves.

By using a ligature or hanging herself. Not by strangling herself with her bare hands, which is impossible because the hands relax once you lose conciousness.

Most suicides are done by strangulation.

No idea.

Determination, of course.

Indeed that was possibly a problem of translation from French, as proposed by "Anonymous". It may be that in French there is no serious distinction between "hanging oneself" and "strangling oneself".